1. Field of the Invention
The present invention relates to projection optical systems, and more specifically though not exclusively, to a projection optical system configured to enlarge and project an image formed by using an optical modulator onto a projection plane.
2. Description of the Related Art
Liquid crystal projectors for forming images by using optical modulators (e.g., liquid-crystal display elements) and projecting the images onto projection planes (e.g., screens) have been variously proposed.
In particular, liquid crystal projectors have become widely used in conferences, presentations, and other scenes as devices that project images in personal computers on a large screen to allow the image to be viewed.
Liquid crystal projectors can be classified into three-panel and single-panel (time division optical modulation method). In three-panel projectors, a plurality of liquid-crystal display elements, one element corresponding to a red, green, or blue light component, are used, and the color light components are combined and then projected onto a screen through a single projection lens set. In single-panel projectors, a single liquid-crystal display element is used, light modulations for three colors (red, green, and blue) are displayed in a time division manner, and an optical image corresponding to each color is formed onto a screen while being synchronized with color changes of light emitted from the liquid-crystal display element.
For such a liquid crystal projector, when an image formed by using a liquid-crystal display element is projected onto a screen, if a letter image or other specific image appears as a double image, resolution and quality are degraded. To avoid the degradation of image quality, it can be necessary in some circumstances to sufficiently combine pixels corresponding to colors over the full area of the screen. Therefore, it can be necessary in some circumstances to sufficiently correct and/or reduce the color shift (lateral chromatic aberration, or chromatic difference of magnification) occurring in a projection lens over the full range of visible light.
These days, a higher definition image is increasingly desired, and this imposes rigid requirements for chromatic aberration. Therefore, the chromatic aberration is corrected or error reduced by using an anomalous dispersion glass material or other similar materials. For example, Japanese Patent Laid-Open Nos. 2001-215411 and 2004-138678 discuss techniques that sufficiently correct and/or reduce chromatic aberration properties by largely using an anomalous dispersion glass material.
However, the techniques discussed in the above patent documents can be insufficient in some cases in terms of size and weight because generally a compact and lightweight lens system is required as a projection lens employed in a liquid crystal projector.